The present invention relates to a circuit for measuring the pulse width of a remote control signal and more particularly to a circuit for accurately measuring the pulse width of a remote control signal.
Recently, in operating household electric appliances such as a television, video cassette recorder, compact disc player, audio tape recorder, etc., the use of the remote controller is becoming common and the audio/video control system which is the combination of the appliances is now being produced and sold. Since each appliance has its own remote controller, users must inconveniently have a remote controller to operate each of their appliances separately, thus requiting numerous controller devices corresponding to the number of appliances. Recently, efforts have been made to combine the functions of all of the remote controllers into one remote control device.
A reconfigurable remote control transmitter is disclosed in the U.S. Pat. No. 4,623,887.
The remote controller of the general household electric appliance has a plurality of keys and converts the remote control signal having a frequency of about 25 to 45 Khz into infrared rays according to the key operation, and transmits the same. The household electric appliance receives the transmitted infrared rays, once again converts the infrared rays back into electric signals, reads the electric signals and carries out the indicated key instructions.
In the remote controller, data of digital signals, i.e., "0" and "1" as shown in FIG. 1 are formed by combining the high signal interval t.sub.a and the low signal interval t.sub.b. The high signal interval t.sub.a has the signal having a carrier of a predetermined frequency (the waveform A of FIG. 1) and the signal of a signal pulse having no carder (the waveform B of FIG. 1). The waveforms A and B of FIG. 1 represent the single/double type remote control signal system in which two high signal intervals t.sub.a within one period t.sub.p constitutes a corresponding digital signal "1", and a single high signal interval t.sub.a within one period t.sub.p constitutes a corresponding digital signal "0". In addition, other conventional types of remote control signal systems include setting the digital signals "0" and "1" by shortening and lengthening the width of the period t.sub.p, setting the digital signals "0" and " 1" by shortening and lengthening the duty ratio of the high signal interval t.sub.a within one period t.sub.p, and setting the digital signals "0" and "1" by combining the aforementioned settings (refer to U.S. Pat. No. 4,623,887).
All of the aforementioned remote control signal systems are formed by combining of the high signal interval and the low signal interval, and are closely related with the waveform width of the remote control signal. Accordingly, to measure the remote control signal, the waveform widths need to be exactly detected.
FIG. 2 is a circuit diagram of a conventional circuit for measuring the waveform width of the remote control signal. In the circuit of FIG. 2, a photodiode 2 receives the infrared ray 1 transmitted from the remote control transmitter and an amplifier 3 amplifies the output of the photodiode 2. The signal S.sub.a (not shown) at point a amplified in the amplifier 3 is received in a first monostable multivibrator 4 which generates a pulse so (not shown ) at point b having a pulse width of the first time constant T1 set by the resistor R1 and the capacitor C1 whenever the input signal exists. The second monostable multivibrator 5 receives the output pulse S.sub.b of the first monostable multivibrator 4 and generates a pulse S.sub.c (not shown ) at point c having the pulse width of the second time constant T2 set by the resistor R2 and the capacitor C2. When the first time constant T1 becomes shorter than the pulse width of the input signal and the second time constant T2 becomes a little bit longer than one period of the input signal, the output pulse S.sub.c of the second monostable multivibrator 5 is detected as being the envelope of the continuous input signal. This output pulse S.sub.c has a pulse width combining the input signal interval and the time constant T2 itself. Accordingly, a signal having the delay time signal T2 along with the waveform width of a high signal interval of the remote control signal appears as an output pulse S.sub.c of the second monostable mutivibrator 5.
The microprocessor 6 receives the inverted output S.sub.b (not shown) at point b of the first monostable multivibrator to count the number of pulses of the received input signal and receives the output pulse S.sub.c of the second monostable multivibrator to measure the waveform width.
However, since the conventional circuit for measuring the waveform width of the remote control signal uses two monostable multivibrators, resistors and capacitors to detect the envelope of the input signal, exact detection of the waveform width is a problem. In detail, the resistor and the capacitor have different resistances and capacitances according to the device characteristics and the environmental temperature changes so that the error is not constant during the measuring of the waveform, and particularly there is the problem of a high error rate in the case of the waveform B of FIG. 1, which has no carrier, so the originally received waveform is not completely reproduced. That is, when the waveform width is calculated in the microprocessor 6, the error due to the delay time T2 that is included during the envelope detection should be subtracted from the reproduced waveform, but since the RC value varies according to the device characteristics and the temperature change, the error value can not be determined to be a constant value.